What are Superluminescent Diodes?
A Superluminescent Diode (SLD) is a novel optoelectronic broadband light source that works based on superluminescence where the entire optical emission spectra of the material get amplified while propagating through the waveguide. An SLD combines the characteristics of both laser diodes (LD) and light-emitting diodes (LED). i.e., it produces high radiant brightness & output power like a laser diode and has a broader emission spectrum similar to an LED. It is also known as a Superluminescent Light-Emitting Diode (SLED).
The construction of an SLD is very much similar to that of a laser diode. It contains both the p-n junction and optical waveguide but lacks the optical cavity structure that provides optical feedback into the system.
The presence of optical feedback in a waveguide results in the formation of resonance modes which will lead to spectral narrowing of the output. So, avoiding optical feedback ensures that both spectral narrowing and lasing action are suppressed in it. The optical feedback is suppressed by incorporating various methods like tilting the facets with respect to the waveguide, using a bent waveguide, embedding an absorption region, and applying antireflection coating on both ends.
Like many laser diodes, an SLD is an edge-emitting diode and has both the p-doped region and the n-doped region. And an active region is sandwiched between them. When it is forward-biased, electric current flow from the p-region to the n-region causes the spontaneous recombination of generated electrons and holes in the system. Hence, photons are generated in the active region which while traveling along the waveguide will undergo Amplified Spontaneous Emission, a process in which the spontaneously emitted photons are amplified by undergoing multiple stimulated emissions to produce a high-intensity optical output. Since this amplification should take place while avoiding optical feedback, the device is designed such that it has a very high single-pass optical gain.
To ensure broadband output, the p-n junction of SLD is designed such that the electrons and holes can be present in various energy levels. Using non-identical multiple quantum wells in SLD can achieve this result.
Optical Coherence Tomography (OCT)
In OCT, the input light is split into a reference beam and a sampling beam. The reference beam is reflected from the reference mirror while the sampling beam gets reflected from various depths of the sample under observation. These two reflected beams produce an interference pattern at the camera where it is captured for further analysis. Usually, input light with low temporal coherence and high spatial coherence is used for OCT, which makes the captured images depth sensitive. The high optical power and broadband nature combined with the high spatial coherence of SLD makes it an ideal light source to obtain high-resolution images in OCT. It is a non-invasive bioimaging technique that is used in various fields like ophthalmology, cardiology, dermatology, etc.
The highly spatially directed output of SLDs makes them suitable for various fiber-coupled operations as the coupling efficiency will be higher. It is also suitable for situations where a high-power broadband light source with high spatial coherence is needed such as for white light interferometry, fiber optic gyroscopes, optical sensing, optical testing, speckle-free illumination, and fiber optic communications.